WO2009157507A1

WO2009157507A1 - Lithium ion secondary cell

Info

Publication number: WO2009157507A1
Application number: PCT/JP2009/061575
Authority: WO
Inventors: 玉腰博美; 川邊啓祐; 東彪; 柴田進介
Original assignee: 日立マクセル株式会社
Priority date: 2008-06-25
Filing date: 2009-06-25
Publication date: 2009-12-30
Also published as: JP2010034024A; US20110111280A1; CN102077404A; KR101268989B1; KR20110031476A

Abstract

A lithium ion secondary cell comprises a negative electrode (2), which has a negative electrode mixture layer that contains a negative electrode active material, a positive electrode (1), which has a positive electrode mixture layer that contains a positive electrode active material, a separator (3), and an anhydrous electrolyte (4), and is characterized in that the negative electrode active material contains a carbon material that has a raman spectrum R value of 0.2-0.8 when excited by a 514.5 nm-wavelength argon laser and has a lattice spacing d₀₀₂ between the 002 planes of 0.340 nm or less, the content of the carbon material is 60% by mass or greater of the total mass of the negative electrode active material, the density of the negative electrode mixture layer is 1.40-1.65 g/cm³, the separator comprises a laminate that contains a porous layer containing a resin with a melting point of 120-140°C, and a porous layer containing a resin with a melting point of 150°C or higher or a porous layer of which main constituent is inorganic particles with a heat resistance temperature of 150°C or higher.

Description

Lithium ion secondary battery

The present invention relates to a lithium ion secondary battery used for various electric devices.

A lithium ion secondary battery, which is a type of nonaqueous electrolyte battery, is widely used as a power source for portable devices such as mobile phones and notebook personal computers because of its high energy density. In addition, due to consideration of environmental issues, the importance of rechargeable secondary batteries is increasing, and in addition to portable devices, they can be applied to automobiles, electric tools, electric chairs, household and commercial power storage systems. Is being considered.

As described above, the characteristics required of batteries vary widely, and various measures are required depending on the application, but in applications that require use at high currents such as power tools, Further improvements in input / output characteristics are required for higher energy density and shorter charging time, that is, for adaptation to high load equipment.

In order to meet such demands, it has been studied to make the electrode active material suitable for a large current load. For example, a carbon material usually used as a negative electrode active material is made amorphous on the surface of graphite particles. It has been proposed to form a composite material having a low crystal carbon coating layer (see Patent Documents 1 to 4).

Japanese Patent Laid-Open No. 6-267531 Japanese Patent Laid-Open No. 10-162858 Japanese Patent Laid-Open No. 2002-42887 JP 2003-168429 A

However, in applications where both charging and discharging are performed with a large current, such as power tools, the reaction at the electrode tends to become non-uniform, and as the heat is repeatedly used, the large amount of heat generated during charging and discharging causes local localized in the electrode. Therefore, it is a problem that the deterioration of the characteristics becomes large as compared with the case where the mobile phone is used in an application where a large current is not required.

Also, the heat generated during charging / discharging may affect battery members other than the electrodes, causing problems. Usually, since the electric tool is used by packing several unit cells, when the temperature inside the unit cell rises due to charging / discharging, heat is accumulated inside the pack and the temperature of the unit cell further increases. As a result, the internal temperature of the battery rises to near the melting point of the separator, the separator gradually clogs, and it becomes impossible to charge and discharge with a large current, and a battery that can maintain reliability over a long period is necessary. It was said.

The present invention can solve the above-mentioned problems, is excellent in charge / discharge cycle life and reliability at a large current, and is suitable for applications such as electric tools that repeat charge / discharge at a large current. I will provide a.

The lithium ion secondary battery of the present invention is a lithium ion secondary battery including a negative electrode having a negative electrode mixture layer containing a negative electrode active material, a positive electrode having a positive electrode mixture layer containing a positive electrode active material, a separator, and a non-aqueous electrolyte. The negative electrode active material has an R value of a Raman spectrum of 0.2 or more and 0.8 or less when excited with an argon laser having a wavelength of 514.5 nm, and an interplanar spacing d ₀₀₂ of 0.300 nm. The following carbon material is included, the proportion of the carbon material is 60% by mass or more with respect to the whole negative electrode active material, and the density of the negative electrode mixture layer is 1.40 g / cm ³ or more and 1.65 g. / Cm ³ or less, and the separator includes a porous layer containing a resin having a melting point of 120 ° C. or more and 140 ° C. or less, a porous layer containing a resin having a melting point of 150 ° C. or more, or inorganic particles having a heat resistance temperature of 150 ° C. or more. The Characterized in that a laminate comprising a porous layer of the body.

According to the present invention, there is provided a lithium ion secondary battery that has little characteristic deterioration due to charge / discharge at a large current, can maintain stable characteristics over a long period of time, and has high reliability even in a relatively high temperature environment. be able to.

FIG. 1 is a cross-sectional view showing an example of the lithium ion secondary battery of the present invention.

Hereinafter, an example of the lithium ion secondary battery of the present invention will be described. An example of the lithium ion secondary battery of the present invention includes a negative electrode having a negative electrode mixture layer containing a negative electrode active material, a positive electrode having a positive electrode mixture layer containing a positive electrode active material, a separator, and a non-aqueous electrolyte.

The negative electrode is formed by applying a negative electrode active material, a conductive powder serving as a conductive additive and a binder containing a binder on a current collector such as a copper foil, and drying to form a negative electrode mixture layer. Obtained by molding. At that time, in order to increase the energy density of the negative electrode mixture layer, pressing may be performed so that the density of the negative electrode mixture layer is 1.40 g / cm ³ or more. On the other hand, the density of the negative electrode mixture layer should be 1.65 g / cm ³ or less in order to make the infiltration of the electrolyte solution into the negative electrode mixture layer uniform and make the reaction inside the negative electrode mixture layer uniform in charge and discharge. What is necessary is just 1.60 g / cm < ³ > or less. For example, the density of the negative electrode mixture layer can be adjusted by adjusting the molding conditions in the pressure molding step in the production of the negative electrode.

Above negative electrode active material, the ratio of the values of the Raman intensity _{I 1580} of around R value of Raman spectrum (Raman intensity _{I 1350} of around 1350 cm ^-1 1580 cm ^-1 when excited with an argon laser with a wavelength of 514.5nm : I ₁₃₅₀ / I ₁₅₈₀ ) is 0.2 or more and 0.8 or less, and a carbon material having a ₀₀₂ plane spacing d ₀₀₂ of 0.340 nm or less is used. The R value is preferably 0.3 or more, and preferably 0.5 or less. Such a carbon material has a large electric capacity, can easily insert and desorb lithium ions on the particle surface, and can handle charging / discharging with a large current, and the reaction with the electrolyte is suppressed to generate heat during charging / discharging. Therefore, even if charging / discharging with a large current is repeated, excellent characteristics can be maintained for a long time. In particular, it is preferable that the negative electrode active material has a BET specific surface area of 1.5 m ² / g or more and 4.5 m ² / g or less because the above effect is easily exhibited. The BET specific surface area of the negative electrode active material is more preferably 2.5 m ² / g or more, and more preferably 3.6 m ² / g or less.

The BET specific surface area of the negative electrode active material referred to in this specification is calculated by measuring the surface area using the BET equation, which is a theoretical formula for multi-layer adsorption, and is expressed by the specific surface area of the active material surface and micropores. is there. Specifically, it is a value obtained as a BET specific surface area using a specific surface area measurement apparatus (“Mosorb HM model-1201” manufactured by Mounttech) using a nitrogen adsorption method.

The carbon material may be used alone as a negative electrode active material, or other carbon materials or other materials may coexist with the carbon material in order to improve the conductivity or increase the capacity of the negative electrode mixture layer. Good. In this case, in order to easily produce the effect of the carbon material, the ratio of the carbon material in the entire negative electrode active material may be 60% by mass or more.

As the other carbon material used with the carbon material, R value and a high carbon material of less than 0.2 crystallinity, that d ₀₀₂ is illustrated like low carbon material 0.340nm greater crystallinity it can. Further, as materials other than carbon materials, elements such as Si and Sn that alloy with Li, alloys of these elements with metal elements such as Co, Ni, Mn, and Ti, oxides of elements that alloy with Li such as SiO, and the like Examples thereof include oxides having a spinel structure typified by Li ₄ Ti ₅ O ₁₂ and LiMn ₂ O ₄ .

The conductive auxiliary agent may be added as necessary for the purpose of improving the conductivity of the negative electrode mixture layer. Carbon black, ketjen black, acetylene black, fibrous carbon may be used as the conductive powder as the conductive auxiliary agent. Carbon powder such as graphite and metal powder such as nickel powder can be used.

Examples of the binder include, but are not limited to, cellulose ether compounds and rubber binders. Specific examples of the cellulose ether compound include carboxymethyl cellulose, carboxyethyl cellulose, hydroxyethyl cellulose, alkali metal salts such as lithium salts, sodium salts, and potassium salts, ammonium salts, and the like. Specific examples of rubber binders include, for example, styrene / conjugated diene copolymers such as styrene / butadiene copolymer rubber (SBR); nitrile / conjugated diene copolymers such as nitrile / butadiene copolymer rubber (NBR). Rubber; Silicone rubber such as polyorganosiloxane; Polymer of alkyl acrylate ester; Acrylic obtained by copolymerization of alkyl acrylate ester with ethylenically unsaturated carboxylic acid and / or other ethylenically unsaturated monomers Rubber; Fluororubber such as vinylidene fluoride copolymer rubber.

The positive electrode forms a positive electrode mixture layer on a current collector such as an aluminum foil by applying a positive electrode active material, a conductive powder serving as a conductive auxiliary agent and a binder, and drying the coating. Obtained by molding.

The positive electrode active material is not particularly limited, but a lithium-containing composite oxide having a spinel structure [a lithium manganese oxide represented by a general formula LiMn ₂ O ₄ (some of constituent elements include Co, Ni, Including complex oxides substituted with elements such as Al, Mg, Zr, and Ti.), Lithium titanium oxides represented by the general formula Li ₄ Ti ₅ O ₁₂ (some of the constituent elements are Co, Ni, Examples include complex oxides substituted with elements such as Al, Mg, Zr, and Ti. ] Lithium-containing composite oxide having a layered structure [a lithium cobalt oxide typified by the general formula LiCoO ₂ (a composite in which some of the constituent elements are substituted with elements such as Ni, Mn, Al, Mg, Zr, Ti, etc.] A lithium nickel oxide represented by the general formula LiNiO ₂ (a part of the constituent elements includes at least one element selected from Co, Mn, Al, Mg, Zr and Ti) And a complex oxide substituted with an element). ], A lithium complex compound having an olivine structure represented by the general formula LiM ¹ PO ₄ (wherein M ¹ is at least one selected from Ni, Co, Fe and Mn) and the like can be preferably used.

In particular, it is represented by the general formula LiNi _1-xy Co _x M ² _y O _{2 in} which a part of Ni in the spinel structure lithium manganese oxide and the layered structure lithium nickel oxide is replaced by Co and the element M ^2. Lithium nickel cobalt composite oxide (wherein M ² is a substitution element containing at least one element selected from Mn, Al, Mg, Zr and Ti, and 0.05 ≦ x ≦ 0.4, 0 ≦ y ≦ 0.5, more preferably 0.1 ≦ x ≦ 0.4, 0.02 ≦ y ≦ 0.5) and lithium composite compounds having an olivine structure are more preferable because of high stability at high temperatures. Used. Examples of the lithium manganese oxide having the spinel structure include Li _{1 + x} Mn _2-xy M ³ _y O ₄ (where M ³ is at least one selected from Co, Ni, Al, Mg, Zr and Ti). Substitutional elements including elements, -0.05 ≦ x ≦ 0.1, 0 ≦ y ≦ 0.3), Li _{1 + x} Mn _1.5 Ni _0.5 O ₄ (−0.05 ≦ x ≦ 0. The lithium nickel cobalt oxide having a layered structure is specifically exemplified as Li _{1 + x} Ni _1/3 Co _1/3 Mn _1/3 O ₂ (−0.05 ≦ x ≦ 0.1), Li _{1 + x} Ni _0.7 Co _0.25 Al _0.05 O ₂ (−0.05 ≦ x ≦ 0.1), and the like are specifically exemplified.

In order to better cope with charge / discharge of a battery with a large current, a lithium cobalt oxide having a layered structure (more preferably, some of the constituent elements are Ni, Mn, Composite oxide substituted with an element such as Al, Mg, Zr, or Ti) or lithium nickel oxide having a layered structure (more preferably, lithium nickel cobalt composite oxide), the proportion of which is 50% of the total positive electrode active material. It is desirable that the amount is not less than 80% by mass. As other active materials, it is desirable to contain a spinel-structure lithium manganese oxide.

The conductive auxiliary agent may be added as necessary for the purpose of improving the conductivity of the positive electrode mixture layer. Carbon black, ketjen black, acetylene black, fibrous carbon may be used as the conductive powder as the conductive auxiliary agent. Carbon powder such as graphite and metal powder such as nickel powder can be used.

Examples of the binder include, but are not limited to, polyvinylidene fluoride and polytetrafluoroethylene.

In the battery of the present invention, the preferable range of the ratio p / n between the mass p of the positive electrode active material and the mass n of the negative electrode active material varies depending on the type of the positive electrode active material. For example, when the positive electrode active material mainly includes a lithium cobalt oxide having a layered structure, the ratio p / n is set to 2.05 or more and 2.30 or less on the surface where the positive electrode mixture layer and the negative electrode mixture layer face each other. Is desirable. In the case where the positive electrode active material mainly contains lithium nickel oxide having a layered structure, the ratio p / n is set to 1.69 or more and 1.90 or less on the surface where the positive electrode mixture layer and the negative electrode mixture layer face each other. Is desirable.

In the battery of the present invention, the ratio PC / NC of the electric capacity PC per gram of the positive electrode active material and the electric capacity NC per gram of the negative electrode active material is a surface where the positive electrode mixture layer and the negative electrode mixture layer face each other. In this case, it is desirable to set it to 0.97 or more and 1.10 or less. By setting the ratio p / n within the above range or the ratio PC / NC within the above range, the ratio of the electric capacity of the positive electrode and the negative electrode can be optimized, and the charge / discharge cycle characteristics can be further improved. Can do.

The electric capacity PC per 1 g of the positive electrode active material is obtained as follows. That is, a model cell having a lithium foil as a counter electrode is manufactured, and the positive electrode is charged to 4.3 V (constant current charging) at a current value of 0.25 mA / cm ² per unit area, and then a constant voltage of 4.3 V is obtained. Charging is continued until the current value decreases to 0.025 mA / cm ² by voltage, and further, 1 g of the positive electrode active material is obtained from the discharge capacity when discharging to 3 V at a current value of 0.25 mA / cm ² per unit area. The per unit discharge capacity is obtained and is defined as the electric capacity PC.

Further, the electric capacity NC per 1 g of the negative electrode active material is determined as follows. That is, a model cell having a lithium foil as a counter electrode is prepared, and the negative electrode is charged to 0.010 V (constant current charging) at a current value of 0.25 mA / cm ² per unit area, and then a constant voltage of 0.010 V is obtained. Charging is continued until the current value decreases to 0.025 mA / cm ² by voltage, and further, the negative electrode active capacity is determined from the discharge capacity when discharging is performed to 1.5 V at a current value of 0.25 mA / cm ² per unit area. The discharge capacity per gram of the substance is obtained, and this is defined as the electric capacity NC.

A porous film formed by laminating a plurality of thermoplastic resin films each containing a thermoplastic resin having a different melting point between the negative electrode and the positive electrode, or a thermoplastic resin film and inorganic particles as main components A porous film formed by laminating the porous film is disposed as a separator.

In general, a single porous film made of polyolefin used in lithium ion secondary batteries is a resin having a melting point near the shutdown temperature so that shutdown occurs at around 135 ° C. while maintaining a certain degree of heat resistance. Is used. However, due to the large strain of the film, when used in power tools, etc., it does not result in shutdown, but the film is likely to shrink or clog due to the heat generated by the battery, resulting in a short circuit or deterioration in characteristics. is there. Further, if the melting point of the resin is increased in consideration of heat resistance, it becomes difficult to cause a shutdown, which causes a problem in terms of safety.

On the other hand, the laminate used as a separator in the present invention contains a resin having a melting point of 150 ° C. or more in addition to a porous layer (low melting point resin layer) containing a resin having a melting point of 120 ° C. or more and 140 ° C. or less that causes shutdown. Because it contains a porous layer (high melting point resin layer) or a porous layer (heat resistant inorganic particle layer) mainly composed of inorganic particles having a heat resistant temperature of 150 ° C. or higher, it is suitable for applications such as electric tools where the internal temperature of the battery is likely to rise. Even if it is used, thermal contraction of the separator is suppressed, clogging is hardly caused, and the characteristics of the separator are stably maintained. For this reason, the characteristic which the negative electrode active material and positive electrode active material which were mentioned above can be exhibited effectively, there is little characteristic deterioration by charging / discharging by a large electric current, and it is a reliable battery also in a comparatively high temperature environment. It can be. The separator may be a high melting point resin layer or a laminate composed of two layers of a heat resistant inorganic particle layer and a low melting point resin layer. In particular, both surfaces have a high melting point resin layer and a low melting point resin layer disposed inside. A laminate of three or more layers, a laminate of three or more layers including a high-melting point resin layer, a heat-resistant inorganic particle layer, and a low-melting point resin layer are suitable for the above purpose, and more preferably used.

The melting point of the resin contained in each layer of the separator referred to in this specification means the melting temperature measured using a differential scanning calorimeter (DSC) in accordance with the provisions of Japanese Industrial Standard (JIS) K7121. Yes.

For the low melting point resin layer, a porous film made of a resin such as polyethylene, polybutene, ethylene propylene copolymer (low melting point resin having a melting point of 120 to 140 ° C.) is used. As the low melting point resin, high density polyethylene having a density of 0.94 g / cm ³ or more and 0.97 g / cm ³ or less is particularly preferable. The low melting point resin layer may contain components other than the low melting point resin. Examples of such components include resins having a melting point other than 120 to 140 ° C. (for example, a high melting point resin described later), inorganic particles contained in a heat-resistant inorganic particle layer described later, and the like. The content of the low melting point resin (resin having a melting point of 120 to 140 ° C.) in the low melting point resin layer is preferably, for example, 80 to 100% by mass with respect to the entire low melting point resin layer.

For the high melting point resin layer, a porous film made of a resin such as polypropylene, poly 4-methylpentene-1, poly 3-methylbutene-1 (high melting point resin having a melting point of 150 ° C. or higher) is used. . Polypropylene is particularly preferable as the high melting point resin. The high melting point resin layer may contain components other than the high melting point resin. Examples of such components include resins having a melting point of less than 150 ° C. (for example, the low melting point resin), inorganic particles contained in a heat-resistant inorganic particle layer described later, and the like. The content of the high melting point resin (resin having a melting point of 150 ° C. or higher) in the high melting point resin layer is preferably, for example, 80 to 100% by mass with respect to the entire high melting point resin layer.

As a porous film formed by laminating a plurality of thermoplastic resin films each containing a thermoplastic resin having a different melting point, for example, a melting point formed by a stretching method or an extraction method is 120 ° C. or higher and 140 ° C. or lower. A porous layer containing the above resin and a porous layer containing a resin having a melting point of 150 ° C. or higher, which is also formed by the stretching method or the extraction method, are overlapped and bonded by stretching, pressure bonding, adhesive, or the like. It is manufactured by a method of forming, or a method in which a layer containing a resin having a melting point of 120 ° C. or more and 140 ° C. or less and a layer containing a resin having a melting point of 150 ° C. or more are thermocompression bonded and made porous by a stretching method or the like. Commercially available laminated films can be used.

The inorganic particles forming the heat-resistant inorganic particle layer are inorganic particles having a heat-resistant temperature of 150 ° C. or higher, that is, inorganic particles having heat resistance that does not show deformation such as softening at least at 150 ° C. Electrochemically stable particles that are difficult to be oxidized and reduced in the battery operating voltage range are preferably used. More specifically, inorganic oxides such as iron oxide, SiO ₂ , Al ₂ O ₃ , TiO ₂ , BaTiO ₃ , and ZrO ₂ ; inorganic nitrides such as aluminum nitride and silicon nitride; calcium fluoride, barium fluoride, Examples include slightly soluble ion-binding compounds such as barium sulfate; covalent bonding compounds such as silicon and diamond; clays such as montmorillonite. Here, the inorganic oxide may be a mineral resource-derived substance such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, mica, or an artificial product thereof. Among the inorganic particles, Al ₂ O ₃ , SiO ₂ and boehmite are particularly preferably used.

The shape of the inorganic particles may be, for example, a shape close to a sphere or may be a plate shape, but is preferably a plate particle from the viewpoint of preventing a short circuit. Typical examples of the plate-like particles include plate-like Al ₂ O ₃ and plate-like boehmite. Moreover, the thing of the secondary particle shape which the primary particle aggregated can also be used suitably. By using particles having a secondary particle shape, adhesion between particles can be prevented to some extent, and voids between particles can be appropriately maintained. Thereby, the path | route which ion permeate | transmits can be ensured, high ion permeability can be maintained, and it can be set as the structure suitable for charging / discharging by a large current.

The particle size of the inorganic particles is an average particle size, preferably 0.01 μm or more, more preferably 0.1 μm or more, preferably 15 μm or less, more preferably 5 μm or less. As used herein, the average particle size of the particles is determined by using, for example, a laser scattering particle size distribution meter (for example, “LA-920” manufactured by HORIBA) and using a particle that does not dissolve these particles (for example, water). It can be defined as the number average particle diameter measured by dispersing.

The heat resistant inorganic particle layer is a porous layer formed by binding the inorganic particles to each other with a resin or binder used in the high melting point resin layer, and the low melting point resin layer or the high melting point resin. Formed on the layer. The ratio of the inorganic particles in the heat-resistant inorganic particle layer may be such that the inorganic particles are 50% by volume or more in terms of solid content so that the inorganic particles are mainly contained. On the other hand, the solid content ratio of the inorganic particles is preferably 99% by volume or less in order to improve the binding property due to the binder.

Examples of the binder include highly flexible resins such as ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, fluorine rubber, styrene-butadiene rubber, carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol, and polyvinyl butyral. Polyvinyl pyrrolidone, cross-linked acrylic resin, polyurethane, epoxy resin and the like are used. In particular, a heat-resistant binder capable of maintaining excellent binding properties up to a temperature of 150 ° C. or higher and maintaining the shape of the heat-resistant inorganic particle layer is preferably used. The heat-resistant inorganic particle layer can be formed by applying a slurry containing the inorganic particles, the binder, and the like dispersed in a solvent to the high-melting resin layer or the low-melting resin layer and drying. .

The thickness of the high-melting point resin layer or the heat-resistant inorganic particle layer is preferably 1 μm or more, more preferably 3 μm or more in order to suppress the thermal shrinkage of the separator, while reducing the thickness of the entire separator. Therefore, it is preferably 15 μm or less, more preferably 10 μm or less. Further, the thickness of the low melting point resin layer is preferably 3 μm or more in order to ensure shutdown, more preferably 5 μm or more, and on the other hand, 20 μm or less in order to reduce the thickness of the entire separator. Preferably, it is 15 μm or less.

The nonaqueous electrolytic solution according to the battery of the present invention is not particularly limited, and a general-purpose nonaqueous electrolytic solution in which an electrolyte salt such as a lithium salt is dissolved in a nonaqueous solvent such as an organic solvent is generally used.

Examples of the non-aqueous solvent include dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propionate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, ethylene glycol sulfite, 1,2-dimethoxyethane, 1, A solvent such as 3-dioxolane, tetrahydrofuran, 2-methyl-tetrahydrofuran, diethyl ether or a mixture of several solvents can be used.

As the electrolyte salt, for _{_{_{example, LiClO 4, LiPF 6, LiBF}}} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) 2, LiN (CF 3 SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (2 ≦ n ≦ 5), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group] and the like. The concentration of the electrolyte salt in the electrolytic solution is preferably 0.3 to 1.7 mol / L, particularly 0.5 to 1.5 mol / L.

In order to further improve the charge / discharge cycle characteristics and storage characteristics of the non-aqueous electrolyte, vinylene carbonate or derivatives thereof; alkylbenzenes such as cyclohexylbenzene and tertiary butylbenzene; cyclic sultone such as biphenyl and propane sultone An additive such as sulfides such as diphenyl disulfide may be contained. The addition amount of the additive may be 0.1 to 10% by mass in the non-aqueous electrolyte, more preferably 0.5% by mass or more, and more preferably 5% by mass or less.

Next, an example of the lithium ion secondary battery of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing an example of the lithium ion secondary battery of the present invention. In FIG. 1, a lithium ion secondary battery includes a positive electrode 1 having a positive electrode mixture layer containing the positive electrode active material according to the present invention described above, a negative electrode 2 having a negative electrode mixture layer containing a negative electrode active material, and a separator. 3 and a non-aqueous electrolyte 4 are provided. The positive electrode 1 and the negative electrode 2 are spirally wound through a separator 3 and are housed in a cylindrical battery can 5 together with a nonaqueous electrolyte solution 4 as an electrode body having a wound structure.

However, in FIG. 1, in order to avoid complication, the metal foil, which is a current collector used for manufacturing the positive electrode 1 and the negative electrode 2, is not illustrated. Moreover, although the separator 3 shows the cut surface, it does not attach | subject the hatching which shows a cross section.

The battery can 5 is made of, for example, iron and nickel-plated on the surface, and an insulator 6 made of, for example, polypropylene is disposed at the bottom of the battery can 5 prior to the insertion of the above-described wound electrode body. The sealing plate 7 is made of, for example, aluminum and has a disk shape. A thin portion 7a is provided at the center of the sealing plate 7, and a pressure introduction port 7b for allowing the battery internal pressure to act on the explosion-proof valve 9 around the thin portion 7a. As a hole. And the protrusion part 9a of the explosion-proof valve 9 is welded to the upper surface of the thin part 7a, and the welding part 11 is comprised. The thin-walled portion 7a provided on the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 are shown only on the cut surface for easy understanding on the drawing, and the contour line behind the cut surface is not shown. is doing. In addition, the welded portion 11 between the thin-walled portion 7a of the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 is also shown in an exaggerated state so as to facilitate understanding on the drawing.

The terminal plate 8 is made of, for example, rolled steel, has a nickel-plated surface, has a hat-like shape with a peripheral edge portion, and the terminal plate 8 is provided with a gas discharge port 8a. The explosion-proof valve 9 is made of, for example, aluminum and has a disk shape. A projecting portion 9a having a tip portion is provided on the power generation element side (lower side in FIG. 1) at the center thereof, and the thin-walled portion 9b is provided. As described above, the lower surface of the protruding portion 9a is welded to the upper surface of the thin-walled portion 7a of the sealing plate 7 to form the welded portion 11. The insulating packing 10 is made of, for example, polypropylene and has an annular shape. The insulating packing 10 is arranged at the upper part of the peripheral edge of the sealing plate 7, and the explosion-proof valve 9 is arranged at the upper part thereof, so that the sealing plate 7 and the explosion-proof valve 9 are insulated. At the same time, the gap between the two is sealed so that the electrolyte does not leak from between them. The annular gasket 12 is made of, for example, polypropylene. The lead body 13 is made of aluminum, for example, and connects the sealing plate 7 and the positive electrode 1. An insulator 14 is disposed on the upper part of the wound electrode body, and the negative electrode 2 and the bottom of the battery can 5 are connected to each other by a lead body 15 made of nickel, for example.

In the battery of FIG. 1, the thin-walled portion 7a of the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 are in contact with each other at the welded portion 11, and the peripheral portion of the explosion-proof valve 9 and the peripheral portion of the terminal plate 8 are in contact. 1 and the sealing plate 7 are connected by a lead body 13 on the positive electrode side. Therefore, in a normal state, the positive electrode 1 and the terminal plate 8 are connected to the lead body 13, the sealing plate 7, the explosion-proof valve 9 and their welded parts. The electrical connection is obtained by 11 and functions normally as an electric circuit.

When an abnormal situation occurs in the battery, such as the battery is exposed to high temperature or generates heat due to overcharge, and gas is generated inside the battery and the internal pressure of the battery increases, the explosion-proof valve 9 The center part of the is deformed in the internal pressure direction (the upper direction in FIG. 1). Along with this, a shearing force is applied to the thin portion 7a of the sealing plate 7 integrated at the welded portion 11, and the thin portion 7a is broken, or the projection 9a of the explosion-proof valve 9 and the thin portion 7a of the sealing plate 7 are broken. After the welded portion 11 is peeled off, the thin-walled portion 9b provided in the explosion-proof valve 9 is cleaved to discharge the gas from the gas discharge port 8a of the terminal plate 8 to the outside of the battery, thereby preventing the battery from bursting. Designed to be able to.

The lithium ion secondary battery of the present invention has little characteristic deterioration due to charging / discharging with a large current, can maintain stable characteristics over a long period of time, and has high reliability even in a relatively high temperature environment. Therefore, the lithium ion secondary battery of the present invention is suitable for applications in which charging / discharging is repeated with a large current or the battery is used in a relatively high temperature environment, such as a power supply application for an electric tool. Moreover, it can be used for various applications to which a conventional lithium ion secondary battery is applied.

Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention.

Example 1
As the negative electrode active material, the R value of the Raman spectrum when excited by an argon laser having a wavelength of 514.5 nm is 0.32, the interplanar spacing d ₀₀₂ of the 002 plane is 0.336 nm, and the BET specific surface area is 3.3 m ² / g graphite powder, carboxymethyl cellulose and styrene / butadiene copolymer rubber as binder, water as solvent, and negative electrode active material, binder and solvent mixed in a mass ratio of 98: 1: 1 Then, a slurry-like negative electrode mixture-containing paste was prepared. The obtained negative electrode mixture-containing paste was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried to form a negative electrode mixture layer, and the density of the negative electrode mixture layer was 1.54 g with a roller. After being pressure-molded to reach / cm ³ , the negative electrode was produced by cutting to a width of 57 mm and a length of 1025 mm.

66.5 parts by mass of LiNi _0.82 Co _0.10 Al _0.03 O ₂ and 28.5 parts by mass of LiMn ₂ O ₄ as a positive electrode active material, 2.5 parts by mass of acetylene black as a conductive assistant, and a binder As a mixture, 2.5 parts by mass of polyvinylidene fluoride was mixed uniformly using N-methyl-2-pyrrolidone as a solvent to prepare a positive electrode mixture-containing paste. After applying the paste on both sides of a positive electrode current collector made of an aluminum foil having a thickness of 15 μm, drying to form a positive electrode mixture layer, and pressing the positive electrode mixture layer with a roller until the thickness reaches 84 μm. The positive electrode was manufactured by cutting to a width of 55 mm and a length of 886 mm.

As a separator, a polypropylene film having a thickness of about 7 μm (high melting point resin layer, polypropylene melting point: 165 ° C.), a polyethylene film having a thickness of about 7 μm (low melting point resin layer, melting point of polyethylene: 125 ° C.), and a polypropylene having a thickness of about 7 μm. A porous laminated film was prepared by laminating a film (high melting point resin layer, polypropylene melting point: 165 ° C.) in this order. The porous laminated film (separator) had a total thickness of about 20 μm and an aperture ratio of 46%.

Next, the separator was placed between the negative electrode and the positive electrode, wound in a spiral shape, and inserted into a cylindrical outer can. On the surface where the positive electrode mixture layer and the negative electrode mixture layer face each other, the ratio p / n between the mass p of the positive electrode active material and the mass n of the negative electrode active material is 1.8, and the electric capacity per 1 g of the positive electrode active material. The ratio PC / NC between PC and the electric capacity NC per gram of the negative electrode active material was 1.01.

A solution obtained by dissolving LiPF ₆ at a ratio of 1.2 mol / L in a solvent obtained by mixing ethylene carbonate and dimethyl carbonate in a volume ratio of 1: 2 in a non-aqueous electrolyte, and further adding 2% by mass of vinylene carbonate. This was injected into the outer can and then sealed to obtain a cylindrical lithium ion secondary battery having a diameter of 18 mm and a height of 65 mm.

(Example 2)
The density of the negative electrode mixture layer is 1.60 g / cm ³ , the thicknesses of the positive electrode mixture layer and the negative electrode mixture layer are adjusted so that the ratio p / n is 1.86, and the ratio PC / NC is 1.04. A lithium ion secondary battery was produced in the same manner as in Example 1 except that.

(Example 3)
When the negative electrode active material was excited with an argon laser having a wavelength of 514.5 nm, the R value of the Raman spectrum was 0.32, the interplanar spacing d ₀₀₂ of the 002 plane was 0.336 nm, and the BET specific surface area was 3.3 m ² / g of graphite powder: 80% by mass and a graphite powder having an R value of 0.08: 20% by mass, and the same amount of ketjen black was used instead of acetylene black as a conductive additive for the positive electrode. A lithium ion secondary battery was produced in the same manner as in Example 1 except for the above.

Example 4
The same amount of LiCoO ₂ was used instead of LiNi _0.82 Co _0.10 Al _0.03 O ₂ of the positive electrode active material, and the same amount of ketjen black was used instead of acetylene black as the conductive additive of the positive electrode. A lithium ion secondary battery was produced in the same manner as in Example 1. The ratio p / n of this battery was 2.18, and the ratio PC / NC ratio was 1.01.

(Example 5)
1 kg of boehmite secondary particles (average particle size: 2 μm) are dispersed in 1 kg of water, and 120 g of styrene-butadiene rubber latex (solid content ratio: 40% by mass) is added and dispersed uniformly to form a heat-resistant inorganic particle layer forming slurry. Was prepared. This slurry was applied to one side of a microporous membrane (low melting point resin layer, thickness 16 μm, porosity 45%) formed of polyethylene having a melting point of 135 ° C. and dried, and a heat-resistant inorganic particle layer having a thickness of 5 μm A laminate comprising a low melting point resin layer having a thickness of 16 μm was prepared. A lithium ion secondary battery was produced in the same manner as in Example 1 except that this laminate was used as a separator.

(Comparative Example 1)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that only graphite powder having an R value of 0.12 was used as the negative electrode active material. The ratio p / n of this battery was 1.8 and the ratio PC / NC was 1.01.

(Comparative Example 2)
A lithium ion secondary battery was fabricated in the same manner as in Example 1 except that a single layer porous film made of polyethylene having a thickness of 25 μm and an aperture ratio of 42% was used as the separator.

(Comparative Example 3)
The density of the negative electrode mixture layer is 1.68 g / cm ³ , the thicknesses of the positive electrode mixture layer and the negative electrode mixture layer are adjusted so that the ratio p / n is 1.8, and the ratio PC / NC is 1.01. A lithium ion secondary battery was produced in the same manner as in Example 1 except that.

(Comparative Example 4)
The density of the negative electrode mixture layer is 1.35 g / cm ³ , the thicknesses of the positive electrode mixture layer and the negative electrode mixture layer are adjusted so that the ratio p / n is 1.8, and the ratio PC / NC is 0.99. A lithium ion secondary battery was produced in the same manner as in Example 1 except that.

(Comparative Example 5)
As a separator, a polypropylene film having a thickness of about 7 μm (high melting point resin layer, polypropylene melting point: 165 ° C.), a polyethylene film having a thickness of about 7 μm (melting point of polyethylene: 105 ° C.), and a polypropylene film having a thickness of about 7 μm (high melting point resin) A lithium ion secondary battery was produced in the same manner as in Example 1 except that a porous laminated film in which layers and a polypropylene melting point: 165 ° C. were laminated in this order was used.

Subsequently, for the lithium ion secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 5, constant current-constant voltage charging with a constant current of 0.75 A and a constant voltage of 4.2 V (total charging time: 2 .5 hours), a constant current discharge (discharge end voltage: 2.5 V) was performed at 1.5 A, and the initial discharge capacity was measured. Next, after the constant current-constant voltage charge, a constant current discharge (discharge end voltage: 2.0 V) is performed at 25 A (discharge rate is about 16 C) to measure a discharge capacity in a large current discharge. The ratio to the discharge capacity was evaluated as a large current characteristic. Furthermore, charging / discharging was performed under the same conditions as the measurement of the initial discharge capacity, and the ratio of the discharge capacity to the initial discharge capacity at this time was evaluated as a capacity recovery rate. The results are shown in Table 1.

In addition, the charge / discharge cycle was repeated with a large current, and the ratio of the discharge capacity at the time of 100 cycles, 200 cycles and 500 cycles with respect to the capacity of the first cycle was measured, and the charge / discharge cycle characteristics were evaluated. The results are shown in Table 2. The charge / discharge cycle characteristics were evaluated under the conditions of charge constant current-constant voltage charge with a constant current of 4 A and a constant voltage of 4.2 V, and discharge with a constant current discharge of 3 A (discharge end voltage: 2. 0V).

Further, the same separators used in the lithium ion secondary batteries of Example 1, Example 5, Comparative Example 2 and Comparative Example 5 were cut into a width of 40 mm and a length of 60 mm, and sandwiched between glass plates from both sides, 130 ° C. A heat resistance test was carried out by allowing it to stand in a constant temperature bath for 1 hour. After the test, the separator was taken out from the thermostatic bath, and the amount of change in the length of the separator in the width direction and the amount of change in the Gurley value were measured.

The Gurley value is an index for evaluating the air permeability of the membrane, and is measured by a method in accordance with JIS P 8117. The Gurley value is indicated by the number of seconds that 100 mL of air passes through the membrane under a pressure of 0.879 g / mm ² .

Next, the ratio of the amount of change to the length in the width direction before the test was determined as the shrinkage rate. Moreover, the relative value of the Gurley value after a test was calculated | required by making the Gurley value before a test into 100. The results are shown in Table 3.

The lithium ion secondary batteries of Examples 1 to 5 are excellent in large current characteristics because the reaction at the electrodes is uniform and the battery configuration can cope with the temperature rise inside the battery due to charge and discharge (Table 1). Even when the discharge exceeded 10C, the battery had little deterioration in characteristics after discharge (Table 1), good charge / discharge cycle characteristics (Table 2), and excellent reliability (Table 3).

On the other hand, in the battery of Comparative Example 1, since the R value of the negative electrode active material is small, it becomes difficult to insert and desorb lithium ions on the surface of the particles, which makes it impossible to handle charging / discharging with a large current, resulting in a decrease in large current characteristics. did. Further, in the battery of Comparative Example 2 using a single layer porous film as the separator, as shown by the increase in the Gurley value at a high temperature, the clogging of the separator gradually proceeds and the charge / discharge cycle characteristics deteriorate. There was also a risk of short circuit due to thermal contraction of the separator, and the reliability was low. In the batteries of Comparative Examples 3 and 4, since the density of the negative electrode mixture layer was not appropriate, the reaction inside the negative electrode mixture layer during charge / discharge became non-uniform, and the charge / discharge cycle characteristics deteriorated. Further, in the battery of Comparative Example 5, although the separator does not shrink, the melting point of the resin contained in the low melting point resin layer is too low, so that when the discharge exceeds 10 C, the characteristics of the separator change, and the discharge capacity Caused deterioration.

The present invention can be implemented in forms other than those described above without departing from the spirit of the present invention. The embodiments disclosed in the present application are merely examples, and the present invention is not limited thereto. The scope of the present invention is construed in preference to the description of the appended claims rather than the description of the above specification, and all modifications within the scope equivalent to the claims are construed in the scope of the claims. It is included.

According to the present invention, it is possible to provide a lithium ion secondary battery that is excellent in charge / discharge cycle life and reliability at a large current and is suitable for applications in which charge / discharge is repeated at a large current such as an electric tool.

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Nonaqueous electrolyte 5 Battery can

Claims

A negative electrode having a negative electrode mixture layer containing a negative electrode active material, a positive electrode having a positive electrode mixture layer containing a positive electrode active material, a lithium ion secondary battery comprising a separator and a non-aqueous electrolyte,
When the negative electrode active material is excited by an argon laser having a wavelength of 514.5 nm, the R value of the Raman spectrum is 0.2 or more and 0.8 or less, and the interplanar spacing d 002 of the 002 plane is 0.340 nm or less. Including carbon materials,
The ratio of the carbon material is 60% by mass or more based on the whole negative electrode active material,
The density of the negative electrode mixture layer is not more than 1.40 g / cm 3 or more 1.65 g / cm 3,
The separator is
A porous layer containing a resin having a melting point of 120 ° C. or higher and 140 ° C. or lower;
A lithium ion secondary battery comprising a porous layer containing a resin having a melting point of 150 ° C or higher or a porous layer mainly composed of inorganic particles having a heat resistant temperature of 150 ° C or higher.
2. The lithium ion secondary battery according to claim 1, wherein the negative electrode active material has a BET specific surface area of 1.5 m 2 / g or more and 4.5 m 2 / g or less.
2. The positive electrode active material according to claim 1, wherein the positive electrode active material includes at least one compound selected from the group consisting of a lithium manganese oxide having a spinel structure, a lithium nickel cobalt composite oxide having a layered structure, and a lithium composite compound having an olivine structure. Lithium ion secondary battery.
The positive electrode active material includes a spinel-structure lithium manganese oxide and a layered lithium-nickel-cobalt composite oxide, and the proportion of the lithium-nickel-cobalt composite oxide is 50 masses with respect to the total positive electrode active material. The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is not less than 80% and not more than 80% by mass.
The ratio PC / NC of the electric capacity PC per gram of the positive electrode active material and the electric capacity NC per gram of the negative electrode active material is set to 0. 0 on the surface where the positive electrode mixture layer and the negative electrode mixture layer face each other. The lithium ion secondary battery according to claim 3, which is 97 or more and 1.10 or less.
The ratio PC / NC of the electric capacity PC per gram of the positive electrode active material and the electric capacity NC per gram of the negative electrode active material is set to 0. 0 on the surface where the positive electrode mixture layer and the negative electrode mixture layer face each other. The lithium ion secondary battery according to claim 4, wherein the lithium ion secondary battery is 97 or more and 1.10 or less.